Monotremes are the most primitive mammals. There are three species of monotremes, the duck-billed platypus (Ornithorhynchus) and two spiny anteaters, or echidnas (Tachyglossus and Zaglossus). These mammals lay eggs; after the babies hatch, the mothers nourish their young with milk. Today, monotremes live only in Australia and New Guinea. The name monotreme means "one-holed," referring to the cloaca, a single hole that serves the urinary tract, anus, and reproductive tract in monotremes.

Reproduction: Monotremes lay tiny eggs (less than 2 cm long) that have a leathery shell. The young get milk from a gland on the mother's belly. The platypus lays her eggs on the bank of a stream, then curls around the eggs to protect them and keep them warm. Spiny anteaters lay a single egg in a temporary protective pouch on the mother's belly.

Anatomy: Monotremes have hair like other mammals, and the females produce milk. They have a low metabolic rate and a temperature slightly lower than that of placental mammals.